I'm pretty sure that it would have to be your target's list. Gaining threat with a target probably follows the same rules as whatever mechanic it is that puts you in combat with a target (e.g. healing/buffing a target puts you in combat with all the mobs your target is in combat with). This is backed up by my personal experiences, though maybe I'll test it tomorrow if no one else has by then.

1) should be the case.
You can see it on a lot of encounter (mag infernals) and old encounter like fankriss/razorgore etc.
When the mobs are in combat, healing is global threat to all mobs in combat.
Same with all buff-spells (thats why they nerfed battleshout tanking).

I'd be interested in any test result you get out of this, as like you, I am also sure that using this mouse-over sunder macro has given me agro on a cc'd mob for when it breaks.

CCed mobs definately don't gain threat from healing - if a hunter pulls with a single auto-shot and traps I can ensure that when it breaks it comes after me with just two or three demo shouts despite plenty of healing being chucked around, enough that if it hadn't been trapped theres no way i'd have held aggro.

Mouse over sunder would work in precisely the same way.

If healing built threat on CCed mobs then it would be much much harder for tanks to pick up mobs after they were CCed. Healers would inevitably die every time a CC was broken.

Not really, there's always taunt, and I can't think of any examples of mobs that could be CCed which couldn't be taunted.

It'd certainly get exciting in case of a taunt resist though.